Optical or conductive mark scanning systems of various speeds and types are well known in the prior art. Recently, high speed mark scanning systems have begun to use line scanning cameras or other high-frequency line scanning techniques to scan mark area or data response areas for detection of data marks on scannable documents. An example of such a system is in U.S. Pat. No. 4,300,123, issued Nov. 10, 1981 to J. McMillin et al. In this system, as in many others, the scannable document includes a plurality of timing marks in a "timing track" used to trigger the system to scan or "read" certain data marks or data response areas.
In any mark scanning system and, particularly, in a high speed mark scanning system, the relationship between each timing mark and the corresponding mark area or, more frequently, corresponding row of mark areas is important for proper scoring and interpretation of the scannable documents. In the type of mark scanning system shown in U.S. Pat. No. 4,300,123, a line scan camera runs at high speed and asynchronously with the travel of the scannable document through the scan or read area. This permits several line camera scans per row of mark areas. A multiple-pixel image of each mark area or data response area can thus be developed. This is accomplished in part by use of a software or firmware (PROM) template that isolates specific pixel image areas, so that these specific pixel image areas can be analyzed for the presence or absence of marks or data responses.
While the use of multiple scan lines per mark area row allows for greater resolution and more accurate detection of whether a data mark or data response has been recorded, reading errors can still occur. The system must accurately read each mark area or data response area that is associated with each timing track. Because this type of line scan system uses a fixed number of scan lines to program the amount of delay from detection of a timing mark until the beginning of the sensing of a mark area, it is important to insure that the document maintains a constant velocity while traveling under the reading head of the scanning system. Otherwise, the pixel image template is not in proper correspondence with areas scanned. Any slippage of the document after detecting a timing mark and occurring before or during scanning of the corresponding mark areas could result in an incorrect interpretation of whether a given mark area is marked or unmarked.
The fastest line scan document scanning systems handle and scan documents at a relatively high rate of speed, i.e. 10,000 or more documents per hour. At such speeds, there is always the possibility of slippage or drag as a document travels through the scanning system, due to the mechanical nature of the document handling mechanisms and the unpredictable surface conditions of documents that have been handled by humans. Such slippage may be either in the fomn of a linear slippage (a momentary slowdown as the document travels through the system) or in the form of skewing or rotational slippage (where one edge of the document travels through the scanning area faster than the parallel edge) While both types of slippage are undesirable, skewing of the document translates into a greater possibility of error in scanning the mark areas located farthest away from the timing mark column. Both linear slippage and skew slippage may be caused by: dirty rollers in the document handling mechanism; improper tension of the rollers in the handling mechanism; mutilated, torn or improperly sized scannable documents; foreign particles in the read area; or improper adjustment of the multi-sheet retard rollers in the document feed mechanism.
While the prior art systems are able to detect and respond to complete jam or document transport failure situations, a momentary slippage or skewing of the scannable document as it travels through the read area can go undetected and may lead to erroneous readings. While such errors have been found to be statistically infrequent in well-maintained equipment, for certain critical applications the misreading of even a small number of documents could have serious consequences, as for example, with career-determining test results. Accordingly, there is continuing need for improvements in methods and systems for checking the velocity of scannable documents in high speed mark scanning systems in order to detect momentary slippages or skewing of such documents and to prevent them from being improperly scored or interpreted.